As the voltage of a single battery unit is relatively low, multiple single battery units need to be connected in series to form a battery group when in application. With regard to each single battery unit in the battery group, their terminal voltages may be inconsistent due to changes of their capacities, their internal resistances, the working conditions, and the environment. During the process of charging and discharging, some battery units may be subject to an overvoltage or an undervoltage, which may adversely influence overall usage and the service life of the battery group. Therefore, it is necessary to equilibrate the terminal voltage of each single battery unit.
For an ultra-capacitor used as an energy-storage element, the voltage for each single unit is also relatively low. In practical applications, multiple single units need to be connected in series to form an ultra-capacitor group. Each single ultra-capacitor unit therein, however, may have an inconsistent terminal voltage because of the changes of their capacities, materials, manufacturing processes, working conditions, and the environment. During the charging process, some ultra-capacitor units may be subjected to an overvoltage, which will influence the service life and the usage of the ultra-capacitor group adversely. Similarly, it is also necessary to equilibrate the terminal voltage of each ultra-capacitor unit.
Capacitors are widely used in various types of power supply. In some cases, they are connected in series. Due to their capacity variance, current leakage variance, and the like, each of the capacitors connected in series has an inconsistent voltage, which readily results in an overvoltage failure of some capacitors. Thus, it is necessary to equilibrate the voltages of the capacitors connected in series.
In practice, an energy-storage element group may consist of two or more groups of battery units or capacitors, and in each group, the number and/or terminal voltages of single units may be different. As a result, it is also necessary to equilibrate the terminal voltages of the two or more groups with different terminal voltages or the terminal voltages of different types of energy-storage element groups according to a proportion during the charging and discharging processes; and it is also an issue needed to be addressed to make the terminal voltage of each single unit in various energy-storage element groups be equilibrated with that of each single unit in other energy-storage element group.
In addition, in one systematic equipment or one district, when various energy-storage elements are distributed dispersely, it is necessary to equilibrate the voltage of each energy-storage element and share the energy proportionally.